The present invention relates to a photonic mixer device with a photosensitive layer, at least two modulation gates and at least two readout electrodes connected to the photosensitive layer, the modulation gates being able to be connected to a modulation device which raises and lowers the potential of the modulation photogates relative to one another and relative to the preferably constant potential of the readout electrodes corresponding to a desired modulation function.
Photonic mixer devices and a method for recording amplitude and phase of an electromagnetic wave are known from the state of the art, for example from DE 198 21 974 A1. The measurement principle known from the state of the art uses a photodetector with a photosensitive layer in which incident photons produce charge carriers and two readout electrodes or readout gates which make it possible to read from the detector a current or voltage signal proportional to the number of incident photons. In addition, modulation gates are provided close to the readout electrodes which are biased with a modulated voltage in order to drive the charge carriers produced in the photosensitive layer to the readout electrodes. A modulated voltage signal is applied to the modulation gates which is correlated with the modulation which was impressed beforehand as intensity modulation on the electromagnetic radiation to be detected. Generally speaking, for practical reasons, the intensity modulation of the incident electromagnetic radiation and the modulation of the modulation gates show the same frequency. The decisive factor is that not only do the modulation of the electromagnetic radiation and the modulation voltage of the modulation gates show the same frequency, but the signals are also coupled phase-locked to one another. The modulation signal can follow a cosine-shaped pattern, but can also have any other periodic or quasi-periodic structure. Therefore, the signal applied to the modulation gates can also be called reference signal. The reference signals which are passed to the two modulation gates display a phase displacement of 180° relative to each other, with the result that the modulation gates cause a potential gradient in the photonic mixer device that drives the produced charge carriers to one or the other readout electrode. The voltage or current signal measured via the readout electrodes is a function of the product of the number of generated charge carriers and the modulation or reference voltage. The measured signal is then essentially proportional to the intensity of the incident electromagnetic wave and its phase displacement in relation to the modulation voltage. If the quadrature component of the incident electromagnetic wave is measured simultaneously with a second mixer element, expediently on the same photonic mixer device, the amplitude and phase information of the incident electromagnetic signal is obtained directly from the two values read from the mixer elements. For this, the modulation voltages applied to the modulation gates of the second mixer element must display a phase displacement of 90° relative to the modulation voltages of the first mixer element.
Corresponding photonic mixer devices are known from the German patent application DE 198 21 974 A1 and reference is made to the complete disclosure of this prior application in as much as the basic mode of operation, the structure and the possible uses of the photonic mixer devices are described therein. The photonic mixer devices disclosed in DE 198 21 974 A1 have at least two modulation gates and at least two readout electrodes in the form of long and narrow, parallel strips. The strip form of the gates or electrodes makes it possible to keep the gate lengths, i.e. the dimension of the gates in the direction of the charge carrier movement, as small as possible and at the same time provide a sufficiently large surface area for the production of charge carriers by the incident electromagnetic radiation. A sufficiently high efficiency of the photonic mixer devices can thus also be achieved for short gate lengths.
The readout electrodes generally have metal contacts which are vaporized or sputtered directly onto the photosensitive layer. The photosensitive layer consists of a semiconductor material, for example p- or n-doped silicon. The semiconductor material is doped complementarily to the remaining wafer in the area beneath the contacts. Thus the readout electrodes are formed by pn diodes. However, photonic mixer devices are also known from the state of the art in which the metal contacts are applied to an intrinsic or continuously doped semiconductor material. The readout electrodes then, because of the metal-semiconductor transition, also display a diode-like, non-ohmic characteristic.
It proves disadvantageous that the readout diodes or diode-like readout electrodes in strip form known from the state of the art display a great capacity which limit the charge conversion efficiency (CCE) when using voltage measurement. The term charge conversion efficiency is used to describe the voltage rise per charge carrier produced in the photosensitive material.
In addition, because of their diode characteristic, the strip-shaped readout electrodes used have the disadvantage that they show increased thermal dark currents, by which the use of a current-measurement method is also affected.
Compared with this state of the art, the object of the present invention is to further develop the known photonic mixer device so that it shows an increased charge conversion efficiency and reduced dark currents.